Method for producing pulp having low lignin content from lignocellulosic material

ABSTRACT

A method for the production of cellulose having a low lignin content characterized by a combination of the measures that
     a) a lignocellulosic material is treated with an aqueous solution comprising a C 1-6 -alcohol and having a pH from 10 to 14 at a temperature of below 100° C., whereupon the aqueous solution, in which lignin split off from the lignocellulose is present in dissolved form, is separated from the solid which represents a material enriched with cellulose and hemicellulose,   b) hemicellulose is removed from the material of a) enriched with cellulose and hemicellulose,   c) the material obtained in b), which is hemicellulose depleted, is treated with an alkali sulfite, an alkaline earth sulfite or ammonium sulfite, in particular Na 2 SO 3  and/or with oxygen in aqueous alkaline solution, whereby lignin dissolves and cellulose having a low lignin content is obtained;
 
as well as cellulose which can be produced by the method.

The present invention relates to a method for producing pulp having alow lignin content from lignocellulosic material.

The conversion of lignocellulose may occur in two fundamentallydifferent ways:

-   1) with the aid of the so-called “Thermochemical Platform”, wherein    the lignocellulose is at first gasified and the synthesis gases are    converted to desired products, and-   2) with the aid of the so-called “Sugar Platform”, wherein the main    interest lies in utilizing the sugars bound in the polymers    cellulose and hemicellulose or, respectively, in utilizing the    polymers themselves.

The present invention follows the second path: The polymeric components(cellulose and lignin) are to be separated from lignocellulose, withchanges as few as possible appearing in said components after theseparation process.

In classical pulpings for the production of pulp, nonselectivedegradation methods are generally applied, wherein lignin and xylan aresimultaneously removed from the lignocellulose material at hightemperatures in the presence of sulphur-containing reagents. The removedlignin is thereby changed significantly in its structure by anellationand an attack by the sulphur-containing reagents and normally can beused only as a combustible. Moreover, high-temperature methods may leadto the formation of heterocyclic substances (e.g., furfural) from sugarswhich necessitates sophisticated recycling methods.

A technological improvement in said field would be the development oflow-temperature methods (i.e., at a temperature of less than 100° C. andbelow), with all substances accumulating in the highest possible purityand the material yield being maximized. This would mean a decisiveprogress, since lignocellulosic material could be sepatated inconsecutive degradations at low temperatures (<100° C.) into theindividual pure substance classes lignin, xylan and cellulose whichsubsequently could be converted into higher-quality products. Inaddition, such methods may constitute an energetically economicalprocess due to the low temperatures.

Various methods of producing delignified cellulose are known fromliterature. In the following, methods are described which have foundindustrial application, namely “Soda pulping processes” (soda process),“Kraft pulping processes” (kraft process), and “Sulfite pulpingprocesses” (sulfite process). Many examples and detailed descriptions ofthe techniques can be found in the “Handbook for Pulp & PaperTechnologies”, 2nd Edition, G. A. Smook, Angus Wilde Publications(1992)“. Therein, the prior art is described based on publicationswhich, in each case, depict the most current improvements andmodifications of said processes.

The “Soda Process”

The soda method was developed in 1851 by Burgess and Watts and refrainsfrom the use of sulphur compounds which are harmful to the environment.Only sodium hydroxide serves as a degradation chemical, for which reasonthe process must be carried out at high temperatures for an adequatedelignification. In comparison to other processes, a high decompositionof carbohydrates takes place so that a lower degree of polymerizationand less tear strength of the cellulose are obtained. In addition, themethod is suitable only for materials which are easy to bleach. In somecases, anthraquinone is added for stabilizing the carbohydrates and fora better delignification (soda-anthraquinone process).

In U.S. Pat. No. 6,946,057 B2, an environmentally friendly process isdescribed, wherein a lignocellulosic material is treated in severalstages with an alkaline buffer solution containing, as main components,metaborates and sodium carbonate. The process is performed at atemperature of 160° C. A main aspect of the method described in thepatent is an effective recovery of the degradation chemicals.

From U.S. Pat. No. 3,954,553, a process is known wherein hardwood istreated for one hour with an alkaline solution containing sodiumhydroxide and sodium carbonate at a temperature ranging from 340° F. to376° F. (from 171° C. to 191° C.). Via the combination of the twochemicals, efficiencies and selectivities are described which arecomparable to those of kraft methods.

As compared to the kraft process, the soda process provides apossibility of obtaining lignin with a very small or without a sulphurcontent. However, in order to achieve the same delignification effect aswith the kraft process, the removal of lignin must be performed at evenhigher temperatures than in the kraft process, whereby lignin isobtained which is condensed to a higher extent. In addition, during theheating of carbohydrates at soda-process temperatures, heterocyclicsubstances such as furfural emerge which must be separated in asophisticated fashion.

the “Sulfite Process”

The first patent for the sulfite process was obtained by BenjaminTilghman in 1867. The first industrial method utilizing said processstarted in Sweden in 1874. In said method, mixtures of different saltsof a sulfurous acid are added in most cases as active reagents. Calciumand magnesium salts are most widely used. By appropriate choice of thecounterions, the process may be operated from a highly acidic to analkaline environment. Also, a two-stage combination of both has provedto be useful in some cases. Classical methods operate in a highly acidic(calcium, pH=1-2) or slightly acidic (magnesium, pH=3-4) medium. Thesulfite thereby chemically modifies the lignin and renders it morereadily soluble in water. Due to its harmfulness to the environment, theprocess is today of secondary importance, however, it is frequently usedfor the production of chemical cellulose, since, in this way, acellulose is produced which is easy to bleach.

In U.S. Pat. No. 8,038,842, a method of fractionating a lignocellulosicmaterial into cellulose, hemicellulose and lignin is described. Therein,a treatment with a vapour-shaped mixture of aliphatic alcohol, SO₂,ammonia and water occurs in a continuous or gradual procedure, theconcentration of SO₂ ranging between 10% by weight and 50% by weight inthe solution. Upon the direct contact of said vapour-shaped mixture witha lignocellulosic material for a duration of between 5 minutes and 3hours at a temperature of between 115° C. and 150° C., a mixture ofcellulose, hemicellulose and lignin is formed, from which cellulose canbe removed.

From U.S. Pat. No. 3,630,832, a controlled process for the production ofsulfite pulp is known, wherein the use of sodium sulfide is avoided. Theprocess is performed at a temperature of more than 145° C. and apH-value of between 8 and 11. Using online analytics, the pH-value isthereby monitored and adjusted appropriately.

From EP 1 375 734, a method for the production of cellulose is known,wherein a lignocellulosic material is treated with magnesium bisulfite,optionally with a surplus of SO₂, at a temperature of between 130° C.and 165° C. in order to remove lignin. The method also involves acombination with a bleaching sequence after the removal of the lignin,which is performed exclusively with reagents which are free fromchlorine.

In DE 103 23 376, a method of delignifying lignocellulosic raw materialsis described, wherein the lignin is removed by the use of sulfites inthe presence of an alkaline component, in particular sodium hydroxide orsodium carbonate, or a mixture thereof, in an aqueous solution, applyingincreased temperature and elevated pressure. Thereby an aqueous sulfitesolution is used at the beginning of degradation, and the temperature israised as far as to the maximum reaction temperature. The alkalinecomponent is added only after the start of the degradation.

Although, in those sulfite processes, the molecular mass of cellulose islargely maintained and a decomposition of lignin is avoided, they havethe major drawback that the lignin obtained may contain a sulphurcontent of approx. 5% (S. Brudin, P. Schoenmaker, J. Sep. Sci. 2010, 33,439-452), which greatly limits any possible applications thereof.

The “Kraft Process”

The kraft process was developed in 1884 in Danzig by Carl Ferdinand Dahl(see, e.g., U.S. Pat. No. 296,935) and involves a method in which ligninis dissolved out of the pulp at an excess pressure of from 5 to 14atmospheres, using a mixture of sodium sulfate, sodium carbonate, sodiumhydroxide and sodium sulfide. The lignin is thereby obtained in theblack liquor as a soluble alkali lignin and has a high sulphur content.Lignin according to the kraft process contains between 1.5 and 3.0% ofsulphur (Marton J. “Lignins; Occurrence, formation, structure andreactions”, 1971, Wiley-Interscience, Eds. Sarkanen K. V. and Ludwig C.H., USA, p 666). In most cases, it is combusted as an energy source forgenerating the heat requirement for the process. Currently, the kraftmethod constitutes the most common method for the production of paperpulp.

From US 2011/0073264 A1, a method of increasing the yield of kraft pulpand of increasing the viscosity of the pulp obtained is known. This isachieved by pretreating a lignocellulose material with hot water orwater vapour at a temperature of 160° C. Remaining lignin is thenremoved by treatment with Na₂S and NaOH under kraft conditions.

In US 2011/0067829 A1, an optimization of the kraft process isdescribed, wherein the sulphur concentration is kept constant during thedigestion of pulp by an optimized method of recycling chemicals.

From U.S. Pat. No. 6,613,192 B1, a modification of the kraft process isknown, wherein eucalyptus chips are fermented by inoculation withwhite-rot fungi, whereby lignin is modified and, as a result, becomesmore readily degradable in the kraft process.

From U.S. Pat. No. 7,824,521 B2, an optimization of the kraft process isknown, wherein a lignocellulosic material is treated with an aqueoussolution containing a further base in order to extract acidic materialfrom the lignocellulosic material. In this way, a portion of thehemicellulose is extracted, whereby the actual kraft process proceedsmore effectively.

From U.S. Pat. No. 7,520,958 B2, a method of producing a modified pulpis known, wherein wood chips are treated with peroxides in an acidicsolution in order to remove 5-20% of the hemicellulose. With thispretreated wood, a kraft pulp containing only between 5% and 9% ofhemicellulose in the dry matter can be produced.

In U.S. Pat. No. 6,153,052 A and U.S. Pat. No. 7,828,930 B2, improvedpulp processes are described, wherein polysulfides are used, by whichuse the yield of pulp can be increased. The polysulfides thereby actprimarily as reducing agents inhibiting undesired “peeling” reactions.

In U.S. Pat. No. 7,828,930 B2 and U.S. Pat. No. 6,143,130, a modifiedmethod of the kraft process is described, wherein polysulfides are usedas active delignification components. Said procedure gives higher yieldof pulp and allows to remove lignin at temperatures which are lower(100° C. to 160° C.) than in the classical kraft process.

From EP 0 468 016, a process for the production of kraft pulp is known,wherein a lignocellulosic material is pre-treated with a charged cookingliquor at a temperature between 20° C. and 100° C. The lignocellulosicmaterial pretreated is this way is heated at a temperature of between135° C. and 155° C. and delignified.

From U.S. Pat. No. 6,576,084 B1, a method of increasing the yield aswell as the viscosity of pulp is known. The technique is based on thepre-treatment of a lignocellulosic material with anthraquinone orpolysulfides at a temperature from 80° C. to 130° C. By that measure, adecomposition of cellulose is avoided. Subsequently, the pre-treatedmaterial is reacted with cooking liquor at a temperature of more than130° C.

An advantage of the above-described kraft process is the relativelysimple possibility of producing a delignified cellulose which can easilybe subjected to further bleaching. A major drawback associated therewithis that the potentially high-quality lignin is suitable for a furtherapplication only to a limited extent, since, after being removed fromthe other raw material components, it is significantly changed in itsstructure by high-temperature condensation reactions. Moreover, thelignin is strongly modified with sulphur via the reaction with sulfide.

Further Delignification Methods

As relatively new concepts, organosolv processes have recently come backinto use. They were described for the first time in the 1970ies.Strategies for an “extended cooking” were then developed, whichessentially was based on increasing the degree of delignification andlowering the bleaching effort. As solvents, alcohols such as ethanol ormethanol were used predominantly, and they were mainly supposed toincrease the solubility of the lignin, whereas acids, alkali, sulfite orsulfide or oxidative reagents continued to act as actual degradationchemicals (H. Hergert, 1998, Developments in organosolv pulping; In: R.A. Young and M. Akhtar, Environmentally friendly technologies for thepulp and paper industry; John Wiley & Sons Inc., New York, 5-68).

Organosolv processes can basically be divided into acidic and alkalinevariants. An acidic process is, for example, the Allcell process, whichwas taken up and developed further by the company LIGNOL (C. Arato, E.K. Pye, G. Gjennestad, 2005, The Lignol approach to biorefining of woodybiomass to produce ethanol and chemicals; Appl. Biochem. Biotechnol.,Vol. 121-124, p. 871-882). As substrates, wood, straw or bagasse areprocessed. The underlying chemical reaction is the autohydrolyticcleavage of hemicelluose at a pH from 2.0 to 3.8, which results from theacetic acid separated from the xylan (conditions: 180° C. to 195° C.,ethanol concentration: 35% by weight to 70% by weight, liquid-to-solidratio: from 4:1 to 10:1, reaction time: 30 to 90 minutes). In this way,cellulose is split off partly in the form of insoluble oligosaccharides,and a large part of the hemicellulose is separated into soluble oligo-and monosaccharides. A partthe pentoses is oxidized into furfural underthe reaction conditions. Lignin is likewise hydrolyzed partly andaccumulates together with the other decomposition products in thecooking liquor, from which the decomposition products are thenextracted. The other non-hydrolyzed portion remains in the solid and maybe hydrolyzed, for example enzymatically, into sugars and fermented intoethanol. The lignin remaining in the solid (20% to 25% of the originalone) accumulates as a fermentation residue and can only be combusted.

It may be said that, in acidic processes, the amount of lignin obtainedis relatively small on the one hand, and the decomposition of lignincannot be decoupled from the decomposition of hemicellulose. Because ofthe relatively poor decomposition of lignin a fibre material emergeswith a residual content of lignin which, in the event of being used as achemical raw material, would require a substantial bleaching effort andis unsuitable for said use. Therefore, primarily endeavoured is the useas a raw material for the production of bio-alcohol, although reportsexist which confirm that the accessibility of the residual lignin in thepulp is relatively high (E. K. Pye, J. H. Lora, 1991, The AllcellProcess—A Proven Alternative to Kraft Pulping, TAPPI Journal March 1991,113-117).

In the past, basic organosolv processes were investigated far less oftenthan acidic ones, since high technical requirements are imposed on therecovery of sodium hydroxide, if high amounts of sodium hydroxide areused, in particular if straw is used as a substrate (Marton & Granzow1982, Use of ethanol in alkaline pulping; WO 82/01568).

In Germany in the 1990ies, the organocell process for pulp cooking wasdeveloped for industrial application (N. Zier, 1996, StrukturelleMerkmale eines Organosolv-Lignins bei Variation der Parameter;Dissertation, Technische Universität Dresden). The process proceeds intwo stages, starting with an alcohol-water impregnation (ratio: 3 partsof alcohol: 7 parts of water) at 110° C. to 140° C. and subsequentcooking at 165° C. to 170° C., with 30% sodium hydroxide and 0.1%anthraquinone based on the dry weight of the substrate. The process issuitable for the degradation of hardwood and softwood as well as forannual plants. The pulp grade is comparable to that of kraft pulp andcould be bleached with oxygen in a chlorine-free way. According tovarious information, the facility was closed soon after start-up due totechnical problems, which, partly, were associated with the recovery ofthe high amount of sodium hydroxide (El-Sakhawy et al., 1996: Organosolvpulping, (3), ethanol pulping of wheat straw; Cellul. Chem. Technol. 30,281-296).

For a cost-efficient biorefinery process which is not aimed for theproduction of bio-alcohol, but at utilizing all the main components oflignocellulose as a chemical or raw material, it is necessary to recovera portion as large as possible of the available lignin. This shouldoccur in a homogeneous product stream with minor contaminations fromdecomposition products from other components.

From WO 2011/014894 and WO 2012/027767, low-temperature methods forseparating said lignin from a lignocellulosic material such as straw,bagasse, energy grasses and/or husks are known. The degradation processaims for a low-temperature process for delignification at below 100° C.,whereupon the material formed, which has been enriched with celluloseand hemicellulose can be treated with at least one carbohydrate-cleavingenzyme. One variant is the use of xylanases, whereby xylan is decomposedselectively and a material strongly enriched with cellulose is formed. Adisadvantage of such a low-temperature method is the lignin content inthe cellulose which is high in comparison to pulp from kraft or sodaprocesses.

Separation of Xylan

In WO 2010/124312, a method for the separation of hemicellulose from amaterial enriched with hemicellulose and cellulose is described, whichmaterial is produced according to WO 2011/014894. Thereby the solidmixture obtained after the degradation is suspended in an acetate bufferand treated with xylanases in order to selectively depolymerize andsolubilize hemicellose. The solid obtained after the separation, whichis strongly enriched with cellulose, subsequently may be used for theproduction of glucose, for example, by a conversion with cellulases.

A chemical method of separating xylan from kraft pulp for the productionof chemical cellulose is described in WO 2005/118923 A1. Thereby, thepulp obtained is treated with an at least 5%, preferably 9%, sodiumhydroxide solution at a temperature of less than 25° C. Thexylan-containing solution which is obtained as in US 2010/0021975 A1 maybe concentrated by nanofiltration and precipitated by adding a mineralacid. As described in AT 503 624, the addition of a mono- or multivalentalcohol may boost said precipitation.

Removal of (Residual) Lignin

For the removal of lignin and, respectively, for the cleavage of lignin,for example, the following methods are known from literature:

From EP 1 025 305, a chemical method for lignin depolymerization isknown. It is based on the catalytic effect of complexed copper incombination with hydrogen peroxide or organic hydroperoxides and iscapable of oxidatively cleaving lignin at temperatures of below 100° C.The complexing agents used thereby are pyridine derivatives. Usingsynthetic lignin models, it has been possible to demonstrate that, ifH₂O₂ is used as an oxidant, a cleavage occurs in the propyl side chainof the lignin molecule, as a result of which the lignin polymerdisintegrates into oligomeric subunits. If the copper system is usedwith hydroperoxides, it is possible to delignify wood. The system basedon H₂O₂ appears to be more readily technically feasible. It has beentested as a bleaching additive during the peroxide bleaching of kraftpulp and has led to an improved degree of delignification. Adisadvantage of such a system is the large consumption of expensivereagents such as, e.g., H₂O₂ or pyridine derivatives, and the structuralchange in the lignin.

In U.S. Pat. No. 4,560,437, it is described that, prior to the actualbleaching process, wood pulp may be subjected to a second degradationunder addition of sodium sulfite, whereby the effectiveness of thefollowing bleaching steps, especially of oxygen bleaching, is supposedto be increased and, hence, costs for bleaching chemicals are supposedto be diminished. The best results could be achieved by using 25 to 30kg of SO₂ (in the form of sodium sulfite) per ton of unbleached pulp ata pH of 8.

In addition, it is described in “Oxidation of Lignin Using AqueousPolyoxometalates in the Presence of Alcohols” (ChemSusChem 2008, 1,763-769) that the delignifying effect of polyoxometalates can beincreased substantially under an oxygen atmosphere in the presence ofaliphatic alcohols. It is concluded that, in the catalytic cycle, alkoxyradicals are formed which initiate the cleavage of lignin.

In EP 0 524 127, the effect of ethanol as an additive in the oxygenbleaching of pulp is addressed in detail. It is shown that, by addingcatalytic amounts of ethanol, the efficiency of delignification and theselectivity can be increased equally.

Also in U.S. Pat. No. 5,609,723, it is described that a higherefficiency of oxygen bleaching can be achieved at the same pulpviscosity, if multivalent alcohols are added to the bleaching solution.Moreover, it is known from U.S. Pat. No. 6,923,887 that the efficiencyof peroxide bleaching can be increased by adding an organic solvent,preferably a C₁₋₂-alcohol.

In U.S. Pat. No. 4,004,967, an increased selectivity of the bleaching ofkraft pulp by adding polyols or monovalent alcohols is likewisereported. However, optimum effects were achieved by adding formaldehyde.

In all the methods described, the quality of the cellulose is consideredto be the only criterion. Thereby, it is neglected that lignin alsoconstitutes a valuable raw material component.

In contrast to the processes described, it is an object of the presentinvention to provide a method wherein, in addition to the isolatedlignin, a material strongly enriched with cellulose and having a lowlignin content (Kappa ≦10) can be obtained without any substantialdepolymerization of the cellulose.

In one aspect, the present invention provides a method for theproduction of cellulose having a low lignin content which ischaracterized by a combination of the measures that

-   a) a lignocellulosic material is treated with an aqueous solution    comprising a C₁₋₆-alcohol and having a pH from 10 to 14 at a    temperature of below 100° C., whereupon the aqueous solution, in    which lignin split off from the lignocellulose is present in    dissolved form, is separated from the solid which represents a    material enriched with cellulose and hemicellulose,-   b) hemicellulose is removed from the material of a) enriched with    cellulose and hemicellulose,-   c) the material obtained in b), which is hemicellulose depleted, is    treated with an alkali sulfite, an alkaline earth sulfite or    ammonium sulfite, such as Na₂SO₃ and/or with oxygen in aqueous    alkaline solution, whereby lignin dissolves and cellulose having a    low lignin content is obtained.

A method which is provided according to the present invention is hereindesignated also as a “method of (according to) the present invention”.

In a method according to the present invention, an organic materialcontaining lignin, for example, annual plants such as (dry) grasses, orparts of grasses, preferably grasses, straw, energy grasses such as,e.g., switch grass, elephant grass or abaca, sisal, bagasse, or atypicallignocellulose substrates such as husks, e.g., lemmas such as ricehusks, particularly preferably straw, energy grasses, bagasse or husks,even more preferably straw or bagasse, e.g., straw such as wheat straw,is used as a lignocellulosic material.

In a method according to the present invention, the content of solids atthe beginning of the delignification process according to measure a)preferably amounts to 3 to 30% by weight of the lignocellulosic materialin the aqueous solution and preferably is provided at a consistency from3 to 30% by weight, in particular from 5 to 20% by weight.

In a method according to the present invention, an aliphatic alcohol,such as a C₁₋₆-alcohol, particularly preferably a C₁₋₄-alcohol such asethanol or isopropanol, is preferably used as an alcohol in order toseparate, according to measure a), lignin from a lignocellulosicmaterial.

In a method according to the present invention, the pH of the solutionmay be adjusted according to measure a) with a base, preferably aninorganic base, for example, a hydroxide such as sodium hydroxide,potassium hydroxide.

The lignin degradation according to measure a) in a method according tothe present invention is performed at a temperature of below 100° C.,preferably from 40° C. to 90° C., particularly preferably from 50° C. to70° C. It has turned out that, if temperatures of more than 100° C. areapplied, condensation products of the lignin will emerge to anincreasing extent, for example, condensed phenolic fragments of theformulae

which are formed by ring condensation reactions. It has been found by 2DNMR analysis that the content of such fragments in lignin isolatedaccording to the present invention is surprisingly significantly lowerthan in comparative lignins which have been produced by high-temperatureextraction, as is shown in Table 1 below:

TABLE 1 Condensed phenolic fragments in lignin, mmol/g Diphenyl inLignin methane 4-0-5 5-5 total Lignin obtained according to the 0.020.06 0.10 0.18 method of the present invention Soda lignin from wheatstraw 0.04 0.09 0.12 0.25 Soda lignin from sarkanda grass 0.36 0.21 0.280.84 Soda lignin from agricultural fibres 0.40 0.25 0.31 0.96 Organosolvlignin from hardwood 0.45 0.25 0.27 0.97

Moreover, sulphur containing reagents do not have to be used in thedegradation process according to measure a) so that the lignin isolatedaccording to the method of the present invention is free from sulphurfrom sulphur-containing reagents.

In one aspect, the present invention is based on the finding that from alignocellulosic material treated with an aqueous basic solutioncomprising an alcohol, in particular a C₁₋₆-alcohol, and having apH-value of from 10 to 14 lignin may be obtained which is lesscondensed. In addition, the treated material which is enriched withcellulose and hemicellulose proves to be a material more readilyapplicable for an enzymatic degradation into carbohydrate cleavageproducts.

An advantage of the method according to the present invention is that aselective isolation of pure lignin is possible as a result of the lowsolubility of hemicellulose at higher alcohol concentrations. In asubsequent step, hemicellulose may be isolated according to measure b)as a pure product from the lignocellulose material enriched withcellulose and hemicellulose.

In a method according to the present invention, approximately 59% to 92%of the total lignin may be removed. This enables a further enzymaticconversion in order to separate hemicellulose, preferably xylan, fromcellulose. Alternatively and in one aspect according to the presentinvention, hemicellulose may be separated by a combination of chemicaland enzymatic treatments in order to obtain dexylanized cellulose whichis as pure as possible.

In contrast, a preceding enzymatic degradation of the xylan has provedto be less effective since the sugars of the lignocellulose are presentin tightly crosslinked, polymeric, crystalline structures of thecellulose and hemicellulose, which, in addition, are enveloped by alignin coat, whereby an extremely dense complex is created. Directaccess to the individual classes of polymers is thereby exacerbated. Dueto their high molecular weights, the enzymes are incapable ofpenetrating into the lignocellulose through the narrow pores. This meansthat a first step must be taken which increases the porosity of thelignocellulose and thereby enables a further enzymatic conversion.

The depletion and optionally isolation of the hemicellulose in a methodaccording to the present invention according to measure b) may occurchemically or enzymatically.

In a further aspect, the present invention provides a method accordingto the present invention in which, according to measure b), either

hemicellulose is solubilized from the material of a) enriched withcellulose andhemicellulose by treatment with at least one carbohydrate-cleavingenzyme, orhemicellulose is solubilized from the material of a) enriched withcellulose andhemicellulose by treatment with an aqueous alkaline solution at atemperature from 20° C. to 50° C., preferably from 25° C. to 35° C., orhemicellulose is solubilized from the material of a) enriched withcellulose andhemicellulose both by treatment with an aqueous alkaline solution at atemperature from 20° C. to 50° C., preferably from 25° C. to 35° C., andby treatment with at least one carbohydrate-cleaving enzyme.

The dissolved hemicellulose may be isolated from the solution byseparating the solution from the solid.

The combined measures a) and b) in a method according to the presentinvention have proved to be particularly suitable for the depletion ofcellulose from a lignocellulosic material.

In comparison to the methods of the prior art, the method according tothe present invention for the production of delignified pulp from alignocellulosic material is a process by which the separate isolation ofall partial streams is rendered possible, whereby a high-qualitysulphur-free lignin is obtained. A significant advantage of the methodaccording to the present invention is a high delignification along withhigh selectivity.

By the delignification according to measure a), which is performedduring the degradation, the porosity of the cell walls of thelignocellulosic material is increased so that carbohydrate-cleavingenzymes can penetrate into the straw and hydrolyze the xylan containedtherein. In addition, activity losses of the enzymes can be reduced bynonspecific bonds to the lignin. Pure endo-1,4-β-xylanases, e.g., enzymepreparations with pure endoxylanase activity, are suitable for theremoval of hemicellulose, in particular xylan, from the lignocellulosematerial, wherein the cellulose is preserved according to measure b) ina method of the present invention. Mixed enzymes with β-xylosidase orα-L-arabinofuranosidase activity convert also cellulose in too largeproportions and therefore must be excluded. For example, Pentopan BG™ aswell as Pulpzyme HC™ are suitable as enzyme preparations with pureendoxylanase activity. Depending on the enzyme, the enzymatic reactionsare performed at optimum conditions and pH-values.

Besides the enzymatic hydrolysis, xylan can also be extracted chemicallyaccording to one aspect of measure b). Thereby the functionalizationwith acidic residues which is high in comparison to cellulose, isexploited. By use of a strong basic solution, xylan may be extractedselectively from a material enriched with cellulose and hemicellulose.In the method according to the present invention, the pH is adjustedaccording to measure b) by an inorganic base, e.g., a hydroxide,preferably sodium hydroxide, with an amount of substance from 1 to 20%by weight, based on the amount of liquid. The extraction is preferablycarried out at a temperature from 20° C. to 50° C., particularlypreferably from 25° C. to 35° C.

A material strongly enriched with cellulose such as, for example, amaterial produced by delignification and dexylanization according to themethod described above is referred to as a pulp, unless otherwiseindicated.

In a method according to the present invention, the material obtainedaccording to measure b), which is hemicellulose depleted, lignin, namelyresidual lignin, is removed from the pulp according to measure c). Forthis purpose, either a sulfite, in particular an alkali, alkaline earthor ammonium sulfite, preferably sodium sulfite and/or oxygen, is used inorder to obtain a pulp with a low content of lignin. It has turned outthat the use of a combination of sulfite and oxygen leads to asubstantially increased efficiency of delignification.

Furthermore, the solution obtained after the removal of the (residual)lignin provides possibilities of isolating and utilizing the removedlignin.

The efficiency of delignification is defined as follows:

${{Efficiency}\mspace{14mu} {of}\mspace{14mu} {delignification}} = \frac{\left( {{{initial}\mspace{14mu} {value}\mspace{14mu} {Kappa}} - {{final}\mspace{14mu} {value}\mspace{14mu} {Kappa}}} \right) \times 100}{{initial}\mspace{14mu} {value}\mspace{14mu} {Kappa}}$

The Kappa value is a measure of the residual content of lignin in thepulp.

The removal of the (residual) lignin in a variant according to measurec) in a method of the present invention using sulfite, e.g., sodiumsulfite, is performed at a temperature from 90° C. to 150° C.,preferably from 120° C. to 140° C.

Surprisingly, it has been ascertained that better delignification occursunder alkaline conditions. Sulfite such as sodium sulfite is preferablyused in an amount from 10 to 100% by weight, particularly preferably of20% by weight, based on the dry matter. An inorganic base, preferably ahydroxide, e.g., sodium hydroxide, at a concentration of from 0.1 to3.0% by weight, based on the dry matter, is used as a base. In this way,an optimum pH from 9 to 13 may be adjusted. In addition, in oneembodiment of the method, an anthraquinone derivative, preferably sodiumanthraquinone-2-sulfonate, may be added as a mediator in an amountpreferably from 0.05 to 5% by weight, particularly preferably in anamount from 0.1 to 1% by weight, most preferably in an amount of 0.5% byweight, based on the dry matter, in order to increase the efficiency ofdelignification.

In a different variant of the method according to the present inventionaccording to measure c), the (residual) lignin can be removed by oxygentreatment at a temperature ranging from 60 to 130° C., preferably from80° C. to 100° C., particularly preferably at 90° C. Thereby aninorganic base, e.g., a hydroxide such as sodium hydroxide, is used inan amount from 1.0 to 10.0% by weight, preferably in an amount of 6% byweight, based on the dry matter. With an appropriate oxygen bleaching,an oxygen pressure, preferably from 1 to 20 bar, particularly preferablyof 10 bar, is applied. In addition, in one embodiment of said method, ananthraquinone derivative, preferably sodium anthraquinone-2-sulfonate,is added as a mediator in an amount from 0.05 to 5% by weight,preferably in an amount of 0.1 and 1% by weight, particularly preferablyin an amount of 0.5% by weight, based on the dry matter. Surprisingly,it has been found that an embodiment in which an aqueous solution with0.1 to 60% by volume of alcohol, preferably from 1 to 5% by volume,particularly preferably with 3% by volume, is used will yield animproved efficiency of delignification.

In order to achieve an intended efficiency of delignification during theremoval of residual lignin of the pulp obtained after the separation ofhemicellulose and lignin, the respective compositions of alldelignification components on a percentage basis as well as the reactionparameters temperature, reaction time and solids concentration can beadjusted.

Celluose produced according to the present invention and having, forexample, the Kappa number and the viscosity which are achievable by themethod according to the present invention is novel and likewise is asubject matter of the present invention.

In a different aspect, the present invention provides cellulose whichcan be produced, and in particular is produced, according to a method ofthe present invention.

By means of the following examples, the impact of the contents ofethanol and mediator as well as of the reaction time on the efficiencyof delignification is illustrated.

In the following examples, all temperatures are in ° C.

EXAMPLE 1 a) Pulping

Wheat straw (21.0% by weight of lignin content, 20.9% by weight of xylancontent, 36.8% by weight of cellulose content, based on the dry matter)is crushed with an ultra-centrifugal mill to a particle size of 2 mm.The crushed straw is suspended in a mixture of water, ethanol (alcoholcontent of 60% by volume) and sodium hydroxide (8% by weight, based onthe dry matter). The mixture is mechanically stirred for 18 hours at aconstant temperature of 70° C. Subsequently, the obtained solid issqueezed and washed with a sufficient amount of water, whereby amaterial enriched with cellulose and hemicellulose is obtained. Theyield and important parameters of the obtained material are illustratedbelow in Table 2.

TABLE 2 Digestion parameters Solid yield 60.1 ± 3.1% Delignification 78± 6% Lignin content in the solid  8.2 ± 1.1% Delignification 75 ± 5%Extracted lignin  15.8 ± 1.18 g per 100 g of wheat straw Extracted sugar 1.4 ± 0.4 g per 100 g of wheat straw

b) Dexylanization (Chemically and Enzymatically)

The material of a) enriched with cellulose and hemicellulose (approx.58.2% by weight of cellulose content, approx. 36.9% by weight of xylancontent, approx. 8.2% by weight of lignin content) is dexylanized in atwo-stage sequence. For this purpose, the material is initiallysuspended in 10% sodium hydroxide, the temperature of which has beenadjusted to 30° C., to a dry matter content of 10% by weight and isstirred at this temperature for 30 minutes. After such time, thehomogeneous mixture is filtered off and washed neutrally with dilutedhydrochloric acid and water. The filter cake obtained is suspended to asolids content of 10%, based on the dry matter, in a 50 mm acetatebuffer solution, whereupon the pH is adjusted to 5. 150 U of PentopanBG™ per g of cellulose is added to said mixture, whereupon thesuspension is heated to 50° C. for 72 hours. Thereupon, the aqueousphase is squeezed and washed with twice the amount of water, whereby thepulp is obtained. A solid yield of 75% of the dry matter used wasobtained. Important parameters of the pulp are compiled below in Table3.

TABLE 3 Parameters composition Cellulose 80.9% (Residual) xylan 2.2% Ash1.0% (Residual) lignin 10.4% (Residual) arabinan 0.7% Other sugars anduronic acids 1.8% MW cellulose 2200 kDa Crystal form Cellulose I + IIKappa number 57.6 CuEn viscosity   694 ml/g Degree of polymerization1850 Degree of whiteness 32.6% ISO

c) Delignification

The pulp of b) was suspended in the water to a dry matter content of 4%by weight. 7.6% by weight of NaOH and 0.5% by weight of Mg²⁺, based onthe dry matter of the pulp, were added to the reaction mixture.Delignification was carried out for 4 hours under an O₂ pressure of 10bar and at a temperature of 90° C. Thereupon, the pulp was washed with asufficient amount of water and dried. Subsequently, the Kappa number andthe viscosity were determined

EXAMPLE 2

Pulping and dexylanization were performed as in Example 1. Theimplementation and the batch of the delignification correspond toExample 1a), except that, instead of water, a 3% aqueous ethanolsolution was used as a reaction medium.

EXAMPLE 3

Pulping and dexylanization were performed as in Example 1. Theimplementation and the batch of the delignification correspond toExample 1, wherein, however, 6.3% by weight of anthraquinone-2-sulfonicacid sodium salt monohydrate, based on the entire dried mass, wasadditionally added as a mediator during the delignification.

EXAMPLE 4

Pulping and dexylanization were performed as in Example 1. Theimplementation and the batch of the delignification correspond toExample 3, except that, instead of water, a 3% aqueous ethanol solutionwas used as a reaction medium.

EXAMPLE 5

Pulping and dexylanization were performed as in Example 1. Theimplementation and the batch of the (residual) delignificationcorrespond to Example 3, except that the reaction time was extended to16 hours.

EXAMPLE 6

Pulping and dexylanization were performed as in Example 1. Theimplementation and the batch of the (residual) delignificationcorrespond to Example 5, except that, instead of water, a 3%ethanol-water mixture was used as a reaction medium.

EXAMPLE 7

Pulping and dexylanization were performed as in Example 1. Fordelignification, the pulp was suspended in the water to a dry mattercontent of 4% by weight and reacted with 64% by weight of sodium sulfiteand 1.5% by weight of sodium hydroxide in the presence of 2.0% by weightof anthraquinone-2-sulfonic acid sodium salt monohydrate, based on thedry matter of the pulp. The reaction time was 1 hour at a temperature of130° C. After delignification, the pulp was washed with a sufficientamount of water and dried. Subsequently, the Kappa number and theviscosity were determined

EXAMPLE 8

The pulp produced according to Example 7 was bleached with oxygen andalkali. The implementation of the reaction and the reaction batch wereas described in Example 4 regarding the delignification.

EXAMPLE 9

Pulping and dexylanization were performed as in Example 1. Theimplementation and the batch of the delignification correspond toExample 7, except that the reaction was carried out for 3 hours at atemperature of 140° C.

EXAMPLE 10

The pulp produced according to Example 9 was bleached with oxygen andalkali. The implementation of the reaction and the reaction batchcorresponded to the data specified in Example 4 with regard todelignification.

The Kappa numbers and viscosity results of the individualdelignification methods are illustrated below in Table 4.

The Kappa value is determined according to DIN 54357 and is a measure ofthe (residual) content of lignin in the pulp. The viscositydetermination was performed in 0.5 m of an aqueous copper ethylenediamine solution (CuEn) with an Ubbelohde capillary viscometer accordingto a simplified test set-up, with the same capillary being used for thesample and the blank value (SCAN-C 15:62, SCAN-C 16:62 and SCAN-CM15:88).

As can be seen in Table 4 below, a considerably stronger reduction inthe Kappa number could be achieved in those trials in which EtOH wasadded right from the beginning. Therein, the efficiency ofdelignification is surprisingly significantly higher than in comparablebleaching processes of kraft pulp.

TABLE 4 Kappa, efficiency of viscosity, mL delignification, % mL/LExample 1 31.8 44.5 699 Example 2 27.8 51.5 740 Example 3 28.3 50.6 717Example 4 24.7 56.9 758 Example 5 18.1 68.4 715 Example 6 14.6 74.5 736Example 7 29.7 48.2 915 Example 8 9.7 83.1 809 Example 9 19.4 66.3 971Example 10 7.7 86.6 843

When the pulp was delignified additionally with Na₂SO₃ prior to theoxygen bleaching, a substantially lower Kappa number of 7.7 ml couldsurprisingly be achieved after two bleaching steps, which, in total,corresponds to a delignification of 86.6%. In comparison to pulpbleached only with oxygen (Example 6), a Kappa number difference ofapprox. 6.9 ml exists. Said difference is significant in view of anindustrial application, considering that, in general, the removal oflignin is more difficult with a low Kappa number and expensive selectivedelignification reagents are required. Moreover, the viscosity of thepulp delignified in a two-stage method is higher than that of a pulpbleached only with oxygen. For example, the pulp produced according toExample 10 was furthermore bleached with peroxyacetic acid. Peroxyaceticacid was used in an amount of 10% by weight, based on the dry matter.The consistency amounted to 4%, the temperature was 70° C. After areaction time of 3 hours, the pulp was washed with a sufficient amountof water and dried. Subsequently, a Kappa number of 1.2 ml was measured.

1. A method for the production of cellulose having a low lignin content,comprising: a) treating a lignocellulosic material with an aqueoussolution comprising a C₁₋₆-alcohol and having a pH from 10 to 14 at atemperature of below 100° C., whereupon the aqueous solution, in whichlignin split off from the lignocellulose is present in dissolved form,is separated from a solid which represents a material enriched withcellulose and hemicellulose, b) removing hemicellulose from the materialof a) enriched with cellulose and hemicellulose to yield a hemicellulosedepleted material, c) treating the material obtained in b) with analkali sulfite, an alkaline earth sulphite, or ammonium sulfite and/orwith oxygen in aqueous alkaline solution, whereby lignin dissolves andcellulose having a low lignin content is obtained.
 2. Method accordingto claim 1, wherein in b) hemicellulose is solubilized from the materialof a) enriched with cellulose and hemicellulose by treatment with atleast one carbohydrate-cleaving enzyme.
 3. Method according to claim 1,wherein in b) hemicellulose is solubilized from the material of a)enriched with cellulose and hemicellulose by treatment with an aqueousalkaline solution at a temperature from 20° C. to 50° C.
 4. Methodaccording to claim 1, wherein in b), hemicellulose is solubilized fromthe material of a) enriched with cellulose and hemicellulose both bytreatment with an aqueous alkaline solution at a temperature from 20° C.to 50° C. and by treatment with at least one carbohydrate-cleavingenzyme.
 5. Method according to claim 1, wherein in c) a temperature from90° C. to 150° C. is applied if alkali sulfite, an alkaline earthsulphite, or ammonium sulfite is used.
 6. Method according to claim 1,wherein in c) a temperature from 60° C. to 130° C. is applied if oxygenis used.
 7. Method according to claim 1 wherein straw, bagasse, energygrasses are used as a lignocellulosic material.
 8. Method according toclaim 1 wherein the lignocellulosic material is provided in the aqueoussolution at a consistency of from 3 to 30% by weight, based on the drymatter.
 9. Method according to claim 1 wherein in c) an anthraquinonederivative is added as a mediator in an amount from 0.05 to 5% by weightbased on the dry matter.
 10. Cellulose produced according to claim 1.11. Method according to claim 1, wherein the material obtained in b) istreated with Na₂SO₃.
 12. Method according to claim 3, wherein thetemperature is from 25° C. to 35° C.
 13. Method according to claim 4,wherein the temperature is from 25° C. to 35° C.
 14. Method according toclaim 5, wherein the temperature is from 120° C. to 140° C.
 15. Methodaccording to claim 6, wherein the temperature is from 80° C. to 100° C.16. Method according to claim 7, wherein elephant grass, switch grass,and/or husks are used as the lignocellulosic material.
 17. Methodaccording to claim 16, wherein lemmas are used as the lignocellulosicmaterial.
 18. Method according to claim 9, wherein the anthraquinonederivative is sodium anthraquinone-2-sulfonate.
 19. Method according toclaim 9, wherein the anthraquinone derivative is added in an amount from0.1 to 1% by weight based on the dry matter.
 20. Method according toclaim 9, wherein the anthraquinone derivative is added in an amount of0.5% by weight based on the dry matter.